The present invention relates to a display apparatus arranged to limit an electric current therein, and more particularly to a display apparatus, such as a CRT, a plasma display, an LED display and a field emission display, having a characteristic that power consumption varies when the brightness of a displayed image varies.
Self-emission type display apparatuses including CRT, a plasma display, an LED display and a field emission display, generally encounter vary in power consumption therein when the brightness of a displayed image varies.
The reason for this lies in that the quantity of electric currents which flow in the display portion is enlarged because light beams must be emitted more intensely and the number of emission times must be enlarged when a bright image is displayed. When a dark image is displayed, the quantity of electric currents which flow in the display portion is reduced because light beams must be weakened and the number of emission times must be decreased.
Under circumstances where saving of the energy resources is required from a global viewpoint, e.g., display apparatuses are required to have large-size screens and display apparatuses as used in vehicles and on portable telephones, power saving has been required for the display apparatus. Power saving realizes advantages for users and attains effects to be obtained in that loads which must be borne by the display portion and the power supply circuit of the display apparatus can be reduced, the size of circuit can be reduced and low-cost elements can be employed. In addition, the lifetime of the display apparatus can be extended.
Therefore, a method has recently been employed which uses a characteristic which is apparent for a self-emission type display apparatus and with which power consumption varies when the brightness of a displayed image varies in order to save power.
Referring to the drawings, an example of the current limiting circuit will now be described in such a manner that a conventional current limiting circuit employed in a plasma display is taken as an example.
FIG. 15 is a block diagram showing an essential portion of the display apparatus having the conventional current limiting circuit. Referring to FIG. 15, reference numeral 1 represents an A/D conversion means for video signals, 2 represents a video-signal processing means, 3 represents a storage means, 4 represents a display sequence control means, 5 represents a panel of the plasma display, 11 represents a means 11 for generating the number of pulses to be retained, 12 and 31 represent multiplying means, 32 represents a smoothing means and 33 represents a pulse controlled-variable calculating means.
The operation will now be. described. The operation which is performed until an image is displayed will now be described in accordance with the flow of a video signal. The A/D conversion means 1 A/D-converts an analog video signal 101 which is supplied to the plasma display to output digital video data 102. The video signal is treated as digital data in a portion following the A/D conversion means 1. The video-signal processing means 2 subjects digital video data 102 output from the A/D conversion means 1 to brightness, contrast, color temperature correction and gamma conversion processes to output video signal data 103 subjected to the video signal process. The storage means 3 has two storage means, which are a first storage means 36 and a second storage means 37, as shown in FIG. 16. Each of the storage means is a field memory or a frame memory. When the display method of the panel is interlace display, the storage means is able to store video signal for two fields. When the display method is progressive display, the storage means is able to store video signal for two frames. Selectors 38 and 39 disposed in front and behind the storage means 36 and 37 are exclusively and independently operated. When the first selector 38 adjacent to the input portion selects the first storage means 36, the second selector 39 adjacent to the output portion selects the second storage means 37. When the first selector 38 adjacent to the input portion selects the second storage means 37, the second selector 39 adjacent to the output portion selects the first storage means 36. If the first selector 38 adjacent to the input portion selects the first storage means 36, the first storage means 36 is being operated in a write mode. Thus, video signal data 103 supplied to the storage means is written on the first storage means 36. In the foregoing period, the second storage means 37 is operated in the reading mode so that sub-field data 104 having a predetermined number of bits is read from the second storage means 37. When the storage means 3 is interposed, video signal data 103 can be converted into a plurality of sub-field data items 104 which are divided in terms of time. Therefore, the storage means 3 is an important element for the plasma display which displays a gray-scale image by performing pulse-width modulation also when the interlace display is performed. The display sequence control means 4 outputs, to the panel 5, sub-field data 104 of the image signal read from the storage means 3 as write data 105 for selecting cells in the panel 5 from which light is emitted. The panel 5 operates writing electrodes and displays the image in accordance with write data 105.
The operation will now be described which is performed until the number of pulses to be retained for generating retaining discharge to cause the panel to emit light is determined, the description being performed along the flows of data of the number of pulses to be retained. A means 11 for generating the number of pulses to be retained generates and outputs data 111 of the number of pulses to be retained which corresponds to sub-fields. A multiplying means 12 multiplies data 111 of the number of pulses to be retained output from the means 11 for generating the number of pulses to be retained and a controlled variable 124 of the number of pulses (to be described later) for limiting the electric current to output data 112 of the number of pulses to be retained after the electric current is limited. The display sequence control means 4 generates pulse-shape data 113 for causing the panel to emit light by the number instructed with data 112 of the number of pulses to be retained after the electric current is limited. The display sequence control means 4 writes a display position on the panel with write data 105 corresponding to the sub-field, and then outputs, to the panel 5, pulse-shape data 113 for causing the panel to emit light. The panel 5 applies pulse-shape waveform to an electrode after data is written on the panel so that only cells written with write data 105 emit light in accordance with pulse-shape data 113 so that an image is displayed.
The operation for limiting the-electric current will now be described. The multiplying means 31 multiplies sub-field data 104 of the video signal output from the storage means 3 and data 112 of the number of pulses for retaining light emission which is output to the panel to perform an equivalent calculation of the quantity of light to be emitted from the panel in one sub-field period. Then, the multiplying means 31 outputs pseudo quantity 131 of light to be emitted which is obtained by the equivalent calculation. Sub-field data 104 of the video signal corresponds to the number of cells which emit light, while data 112 of the number of pulses to be retained corresponds to the number of light emitting operations of each cell. The smoothing means 32 subjects the pseudo quantity 131 of light to be emitted which varies in sub-field units to a smoothing process to output a smoothed quantity 132 of light to be emitted. The pulse controlled-variable calculating means 33 causes a deviation detection means 34 to obtain deviation 133 of the quantity 132 of light to be emitted with respect to a target value 122 of the quantity of light to be emitted. The deviation 133 is linearly converted into a controlled variable 124 of the number of pulses by a linear conversion means 35 in such a manner that the quantity of light to be emitted is limited only when the quantity 132 of light to be emitted is larger than the target value 122. The controlled variable 124 is output from the pulse controlled-variable calculating means 33. The controlled variable 124 of the number of pulses is expressed by a decimal fraction not less than 0 nor more than 1. The controlled variable 124 reduces the number of pulses to be retained in the multiplication with the data 111 of the number of pulses to be retained which is performed by the multiplying means 12. Therefore, the controlled variable 124 corresponds to the quantity of electric current which must be limited with which the electric current is limited by a larger degree in proportion as the controlled variable 124 is reduced. When the value of the controlled variable 124 of the number of pulses is small, the number of pulses to be retained is reduced by the multiplying means 12. Thus, the number of light emission times from the panel is reduced, and the displaying current which flows in the panel is limited.
The conventional current limiting circuit adapted to the plasma display encounters time delay when the pseudo quantity 131 of light to be emitted is smoothed by the smoothing means 32 as shown in FIG. 15. FIG. 18 shows time transition of the pseudo quantity 131 of light to be emitted which takes place in the conventional circuit. As shown in FIG. 18, the pseudo quantity 131 of light to be emitted is obtained in sub-field units. Even if a still image is being displayed, the quantity of light to be emitted from adjacent sub-fields is discontinuous. Therefore, the smoothing process must be performed by the smoothing means 32. If the smoothing process is simply performed by a low-pass filter, the smoothing process, however, inhibits measurement of the quantity of light to be emitted in field units. As a result, time delay takes place. Even if the quantities of light to be emitted are integrated in field units as shown in FIG. 18, the quantity of light to be emitted cannot be extracted only after the integration of the quantity of light to be emitted is completed and movement to a next field is performed. Thus, time delay of one field takes place in performing the control. If the control is delayed as described above, an excessively large current flows in a case where the quantity of light to be emitted is rapidly increased. In this case, a power supply circuit having a large capacity is required: Thus, increasing cost and limiting the ability to reduce weight.
Since limitation of the electric current results in the brightness varying, the visual characteristic must be considered. For example, overshooting and slight variation of the controlled variable results in flickers being recognized. Therefore, the current limiting circuit for the plasma display must have high response speed and must avoid noise caused variations. The response characteristic of a current control system of the conventional structure depends on the filtering characteristic of the smoothing means 32. If the smoothing means 32 comprises a simple low-pass filter, raising of the response speed results in flickers taking place. The flickers take place because of an influence of variations of the pseudo quantity 131 of light to be emitted in each sub-field and an influence of variation in the waveform of the pseudo quantity 131 of light to be emitted occurring as time elapses attributable to movement of a figure of the bits to the right or left. Another problem arises in that varying the setting of the filter requires a great labor. Even if the quantity of light to be emitted is integrated in field units, the time delay of the process encounters mismatch between the quantity of light to be emitted and the controlled variable. Thus, there arises a problem in that flickers take place.